The ontogeny of perithecia in Guignardia bidwellii was studied with contaminated berries and under in vitro conditions. The first developmental stage consists of a stromatic envelope surrounding a carpocentrum in which an ascogonial apparatus (composed of ascogonial cells and trichogynes) differentiates at the second stage. Stage 3 is marked by the incipient formation of the ascal locule and development of pseudoparaphyses. At the subadult stage ( = stage 4) these filaments intermingle with asci and they are progressively stretched owing to ascus expansion. The mature ascocarp (stage 5) contains numerous eight-spored asci and opens through an apical ostiole. The developmental features of Guignardia bidwellii indicate a pseudoparaphysate ascolocular type of development. Consequently, the fungus is more correctly placed in the Botryosphaeriaceae than in the Mycosphaerellaceae.